### Теория:

A closed line consisting of all points on the plane that are equidistant from a given point on the plane is called a circle.
• In the definition, the given point on the plane is called the center of the circle. Usually, we denote the center by the letter $$O$$.
• The circle is called the circle along with the part of the plane it delimits.
To draw a circle, take a compass with a pen/pencil attached to it and place the sharp end of the compass on a particular point of a paper, then keeping the abrupt end non-movable rotate the compass $$360$$ degrees, so the pen/pencil draws a perfect circle.

The circle, in the drawing, is black. The black line, together with the shaded area forms a circle. Where, $$O$$ is the center of the circle.
• The segment joining the center to a freely chosen point on the circumference is called the radius. We usually denote radius by the lowercase $$r$$ or the uppercase $$R$$.
• The  segment that joins the two points of the circle is called a chord.
• The chord passing through the center of the circle is called the diameter.
$d=2R$ or $R=\frac{d}{2}=0.5\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}d$, where $$d$$ is the diameter of the circle and $$R$$ is the radius of the circle.

$$AO$$$$=$$$$BO$$$$=$$$$EO$$$$=$$$$FO$$ as circular radii.
The radii $$EO$$ and $$FO$$ form the diameter $$EF$$.
$$BC$$ is a chord.